Current-controlled polarization switching vertical cavity surface emitting laser

ABSTRACT

A fast current-controlled polarization switching VCSEL with two independent intra-cavity p-contact electrodes and two independent intra-cavity n-contact electrodes positioned along the four sides of the symmetric aperture such that there are two independent p- and n-contact pairs placed on opposite sides of the aperture in a non-overlapping configuration. The anisotropy resulting from the unidirectional current flow causes the light output to be polarized perpendicular to the direction of current flow. A VCSEL driver circuit switches the polarization state of the output light by using the two orthogonal pairs of non-overlapping intra-cavity contacted electrodes to change the direction of current flow into the VCSEL aperture by 90 degrees.

A CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 60/417,884, filedOct. 11, 2002 to Husain et al. and entitled “Current-controlledpolarization switching VCSEL,” which is incorporated herein by referencein its entirety and for all purposes.

[0002] This application also claims the benefit of priority under 35U.S.C. § 119(e) from the U.S. Provisional Patent Application No.60/______ (Attorney Docket No. 098701-0306365), filed Oct. 10, 2003 toKrishnamoorthy et al. and entitled “Driver for Polarization-SwitchingVCSEL and Method of Operation,” which is incorporated herein byreference in its entirety and for all purposes.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates generally to solid state lasers,and more particularly relates to the polarization switching of verticalcavity surface emitting lasers, or VCSELs.

[0005] 2. Description of the Related Art

[0006] Optical interconnections and transceivers are currently beingused to provide reliable interconnections between electronic componentsthat can scale in both distance and speed. The VCSEL technology has hada substantial impact on this industry as a low-cost, wafer-scale, andhigh-speed device that can be directly driven by low-cost siliconcircuits. For reasons of manufacturing cost, packaging costs andperformance, current-injection (or current-modulation) VCSELs havedominated the low-cost, short reach markets for transceivers in the 1-10Gigabit per second (Gbit/s) range. However, these conventionalcurrent-injection VCSELs are, for numerous reasons, bandwidth limited.

[0007] For example, the current-injection VCSELs exhibit RC limits dueto charging and discharging of the VCSEL and electrode (i.e., thecontacts) capacitances. Electrode capacitance can be eliminated bysuitable design of non-overlapping intra-cavity contacts placed onopposite sides of the cavity to avoid lateral electrode overlap.However, VCSEL intrinsic capacitance (i.e., the active-layercapacitance), to date, cannot be easily reduced. While the intrinsicbandwidth (f_(max)) of VCSELs, based on their fundamental materialproperties, can theoretically be in excess of 90 Gigahertz (GHz), the RClimits due to the charging and discharging of the VCSEL and electrodecapacitances have limited the operating bandwidth to about 20 GHz foreven the fastest conventional VCSELs. Additionally, the conventionalcurrent-injection VCSEL experiences detrimental carrier transporteffects related to movement and re-distribution of carriers in activeregion. Further, heating effects caused by current modulation reducesthe intrinsic bandwidth (f_(max)) of a VCSEL. Finally, mode-competitionnegatively effects the multi-modal VCSELs.

[0008] Conventional VCSEL structures also typically have randompolarization states. Much work has been done in the art to attempt tounderstand the dynamics of the polarization behavior of VCSELs. In atypical prior art VCSEL, as the injected current is varied, thepolarization state can exhibit hysteresis and noisy behavior. This makesthe polarization state of the output light difficult to predict andcontrol. However, it is known that one may “fix” the polarization stateof a conventional VCSEL by introducing an asymmetry into the cavitystructure through the use of mechanical strain. Mechanisms used to applythe strain in different directions in order to switch the polarizationhave been proposed. However, due to the need to mechanically alter thestress, these proposed switching mechanisms are inherently slow.

[0009] It is also known in the art that by designing a VCSEL with arectangular aperture, the polarization state of the output light prefersto align along the direction of the longer axis of the rectangularaperture. Further, polarization switching VCSEL designs based on theintersection of such rectangular aperture regions and switching currentflow along the corresponding longer axes have been proposed. Aside fromeven more complex aperture geometries and associated processingcomplexities, such designs necessitate a more substantial movement andre-distribution of carriers in the active region of the VCSEL, whichtends to reduce the maximum rate of switching between the preferredpolarization states.

[0010] It has also been observed that it is possible to substantiallyfix the polarization of a square cavity VCSEL by using asymmetricnon-overlapping electrodes to preferentially inject current along onelateral axis of the cavity. The proposed structure used a multi-layerDistributed Bragg Reflector (DBR), and one generalized embodiment isshown in FIG. 1. As shown in FIG. 1, both top and bottom DBR mirrors areused, which makes the fabrication of such a VCSEL with one pair of dualintra-cavity contacts difficult and the fabrication of such a VCSEL withtwo pari of dual intra-cavity contacts extremely difficult.

[0011]FIG. 2 illustrates another generalized example of a prior artVCSEL structure. As shown in FIG. 2, the VCSEL structure includes onepair of intra-cavity contacts (i.e. one p-contact 202 and one n-contact204). A top mirror 206 may be a deposited dielectric for maximumprocessing flexibility and VCSEL reliability. This contact is in starkcontrast to the top DBR mirror used in the device shown in FIG. 1. Thebottom mirror 208 of the FIG. 2 device can be epitaxially grown and is asemiconductor DBR. The n-contact 204 is disposed on the bottom mirror208. The VCSEL mesa includes the active quantum-well region 210, acurrent confinement aperture 212, and the remainder of the opticalcavity 214. The p-contact 202 is disposed on top of the VCSEL mesa. Thedielectric top mirror 206 is placed as the last step of the fabricationprocess, completing the cavity structure. The pair of contacts 202, 204are intra-cavity because they bypass the mirror pairs.

[0012] Therefore, what is needed is a VCSEL that provides fast switchingof the polarization state without the limitations of the conventionalart.

SUMMARY OF THE INVENTION

[0013] The present invention achieves very fast switching of thepolarization-state of the output laser light. The high finesse of thevertical cavity laser enhances small anisotropies in the device, whichare caused by injecting current along a uni-axial direction into theaperture and result in the polarization state of the device being fixedorthogonally to the direction of current flow. In an exemplary designintended to achieve polarization switching with approximately equalpower outputs in both polarization states and with a high polarizationcontrast ratio, the aperture typically is small and symmetric, as in,for example, a rectangle. Two orthogonal current paths are established.The anode and cathode of a first current path are positioned along afirst pair of opposite sides of the aperture, while the correspondinganode and cathode of a second, independent current path are positionedalong the remaining pair of opposite sides of the aperture.

[0014] The present invention enables the creation of, along with otheraspects, a single-mode, current-injected semiconductor laser with veryhigh-modulation speed (i.e., in excess of 40 Gbit/s and potentially theintrinsic bandwidth (f_(max)) of the VCSEL) that can be useful in, forexample, optical communication, optical imaging and optical sensingapplications.

[0015] Other advantages of various implementations of the presentinvention include, but are not limited to (nor intended to be limitedby):

[0016] the ability to decouple the laser turn-on current from theswitching mechanism resulting in a low (i.e., near-zero) chirp;

[0017] the ability to provide a constant-current, constant electricalpower device, thereby reducing the difficulty of thermal dissipation,and packaging;

[0018] the ability to achieve high-speed and high extinction ratiosimultaneously in a VCSEL because bias points are decoupled from theextinction ratio (a feature not available with a conventionalcurrent-intensity modulated VCSELs);

[0019] the ability to enable link operation in single-ended mode byconverting polarization modulation to intensity modulation; oralternatively, to allow link operation naturally in differential mode(if both polarizations are carried to separate receivers); and

[0020] the ability to decouple the aperture size of the VCSEL from itsmodulation speed. This offers a more reliable high-speed design becausecurrent density can be reduced versus conventional high speed VCSEL dueto the fact that peak currents are lower and because larger aperturedevices can be used.

[0021] Yet another feature of at least some embodiments of the presentinvention are that wavelength is naturally stabilized, such that currentand temperature do not change as a function of modulation speed,modulation format, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] These and other advantages, aspects and features of the presentinvention will become apparent to those ordinarily skilled in the artupon review of the following description of specific embodiments of theinvention in conjunction with the accompanying figures, wherein:

[0023]FIG. 1 shows a generalized structure of the conventional VCSEL,wherein an intra-cavity contacted VCSEL uses both bottom and topepitaxially grown DBR mirrors;

[0024]FIG. 2 shows a second generalized structure of the conventionalVCSEL that has an intra-cavity contacted VCSEL with a bottom DBR mirrorand a top deposited dielectric mirror;

[0025] FIGS. 3A-B show an exemplary arrangement according the presentinvention with dual pairs of intra-cavity contacts made on a VCSEL withone DBR mirror and one deposited dielectric mirror;

[0026]FIG. 4 shows an exemplary arrangement according to the presentinvention with four pairs of intra-cavity contacts made on a VCSEL;

[0027]FIG. 5 shows another exemplary arrangement according to thepresent invention with four pairs of intra-cavity contacts made onseparate neighboring VCSELs;

[0028]FIG. 6 shows a generalized circuit structure according to anembodiment of the present invention;

[0029]FIG. 7 shows a first embodiment of the VCSEL driver circuitaccording to the present invention;

[0030]FIG. 8 shows a second embodiment of the VCSEL driver circuitaccording to the present invention;

[0031]FIG. 9 shows a third embodiment of the VCSEL driver circuitaccording to the present invention; and

[0032]FIG. 10 shows a fourth embodiment of the VCSEL driver circuitaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] The present invention will now be described in detail withreference to the drawings, which are provided as illustrative examplesof the invention so as to enable those skilled in the art to practicethe invention. Notably, the figures and examples below are not meant tolimit the scope of the present invention. Where certain elements of thepresent invention can be partially or fully implemented using componentsknown in the art, only those portions of such known components that arenecessary for an understanding of the present invention will bedescribed, while detailed descriptions of other portions of such knowncomponents will be omitted so as not to obscure the invention. Further,the present invention encompasses present and future known equivalentsto the known components referred to herein by way of illustration.

[0034] An embodiment of the present invention is shown in FIGS. 3A-B,which illustrate, respectively, a side elevation view and a perspectiveview of the invention. As shown, this embodiment of the presentinvention entails the use of two pairs of orthogonal, lateral-injection,contact electrodes, 302A,B & 304A,B, with a symmetric VCSEL cavity 305and a dielectric top mirror 306 as a means for polarization switchingvia current steering. In an exemplary arrangement, the contact pairs302A,B & 304A,B are arranged on opposite sides of the cavity so as tofacilitate uniaxial flow of current across the cavity. The arrows ofFIG. 3A illustrate this unidirectional current flow betweencorresponding contacts (i.e., 302A,B in this case). Polarizationswitching is achieved by changing the direction of this uniaxial currentflow.

[0035] The use of a relatively thin deposited dielectric mirror 306allows both a current aperture 312 of predetermined size and the twopairs of intra-cavity contacts, 302A,B & 304A,B, to be accuratelypositioned with traditional photolithographic processes. The precisethickness of the dielectric mirror 306 is not a critical dimension ofthe present invention because current does not flow through thedielectric mirror 306, but flows under it and directly into theaperture. Thus, an aspect of the present invention is that thedielectric mirror 306 can be thinner than in the typical design oftoday. For example, in an 850 nanometer (nm) emitting VCSEL, thedielectric mirror 306 of the present invention can be on the order of 1micron or less (or even omitted from the design altogether), while inthe typical design the mirror (i.e., 206 of FIG. 2) will be at least 3microns thick. Because the current flow “bypasses” the dielectric mirror306 and does not have to flow vertically down (i.e. through) arelatively thick mirror, the direction of flow of current is morecontrolled (i.e., planar and unidirectional), which leads to the moreimproved polarization control of the emitted light.

[0036] By using this direct current injection into the aperture 312(which can be much less than 1 micron thick), the present inventionenables precise polarization control to be successfully achieved. Thisprocedure avoids the need to define the aperture size by etching anarrow high-aspect ratio mesa or by using proton-implantation throughpotentially much thicker top-DBR mirror layers, and thus simplifies thefabrication process of the VCSEL. This type of mirror in smalldimensions offers greater reliability, including a substantially reducedtendency to delaminate or degrade. The resulting VCSEL has theadvantages of high-speed, high extinction ratio, and excellentreliability, attributes that are not simultaneously achievable inconventional current-intensity modulated VCSELs.

[0037] Manufacturing the VCSEL of the present invention is performedusing typical semiconductor fabrication techniques as are well know inthe art. An overview of such fabrication will now be provided. First, asemiconductor DBR 308 is fabricated on a substrate. Such a semiconductorDBR is known in the art. In this example, the semiconductor DBR 308 isapproximately circular, with a diameter that can be of any size tosupport the remaining exemplary features. However, other geometricshapes can be used and the present invention is not intended to belimited to using a circular semiconductor DBR 308. The size of thesemiconductor DBR 308 is not critical to the present invention; it couldbe the entire wafer or it could be selectively deposited in certainareas of the wafer.

[0038] Next, an active region (e.g. quantum well region) 310 iscentrally disposed on top of the semiconductor DBR 308 using techniquesthat are known in the art. The active region 310 is also approximatelycircular and can have a diameter of from about 3 microns to over 20microns, with typical values being between about 8 and 12 microns. Aswith the semiconductor DBR 308, other geometric shapes can be used forthe active region 310 and are within the scope of the present invention.Part of the active region 310 includes the oxidized current aperture 312and symmetric VCSEL cavity 305. Also disposed on top of thesemiconductor DBR 308 are two n-contacts 302B, 304B, both of which areapproximately square pads and about 10 to 50 microns on a side. However,the shape and size of the pads can vary and depend on the aperture sizeand capacitance targets, among other variables. The n-contacts 302B,304B are place near the outside edge of, and adjacent to, the activeregion 310 and are spaced approximately 90 degrees apart.

[0039] On top of the active region 310, a dielectric DBR 306 iscentrally disposed and is also approximately circular and about 20 to100 microns in diameter, with different shapes and sizes possible. Sucha dielectric DBR is known in the art. Also disposed on top of thedielectric DBR 306 are two p-contacts 302A, 304A, both of which areapproximately square pads and about 10 to 50 microns on a side. However,the shape and size of the pads can vary and depend on the aperture sizeand capacitance targets, among other variables, as with the n-contacts302B, 304B. The p-contacts 302A, 304A are placed near the outside edgeof, and adjacent to, the dielectric DBR 306. The p-contacts 302A, 304Aare approximately 90 degrees apart from one another, and alsoapproximately 180 degrees apart from the corresponding n-contacts 302B,304B, respectively.

[0040] It is worth noting again that the sizes and shapes of the partsthat make up this embodiment of the VCSEL of the present invention arefor example only. It will be readily apparent to those skilled in theart that there are many variations to these sizes and shapes, all ofwhich are intended to be within the scope of the present invention.Further, the materials used for the VCSEL according to the presentinvention are the ones commonly used to create a VCSEL for anyparticular emission wavelength. For example, creating an 850 nm VCSELmight require GaAs & AlGaAs, while longer wavelength emissions might usea variety of other materials. The present invention is independent ofwavelength, and therefore does not require the use of any particularmaterials.

[0041] In operation, current is supplied (not shown) to the two pair ofindependent intra-cavity p-contact electrodes and n-contact electrodes,the electrodes being positioned pairs along opposite sides of theaperture in a non-overlapping configuration. The anisotropy resultingfrom the unidirectional current flow of one electrode pair causes thelight output to be polarized perpendicular to the direction of suchcurrent flow. Switching of the polarization state of the output light isachieved by using these two orthogonal pairs of non-overlappingintra-cavity contacted electrodes to switch the direction of currentflow in the aperture VCSEL by 90 degrees (i.e., by switching the currentflow from one pair to the other pair, the current flow within the VCSELis switched by ±90 degrees, thereby switching the polarization state ofthe output light).

[0042]FIG. 4 illustrates another embodiment of the present inventionthat allows a VCSEL to emit in one of multiple linear polarizations. Asshown in FIG. 4, multiple pairs of electrodes, 402A-405A & 402B-405B,corresponding to multiple pairs of p-n contacts may be placed around theaperture. This enables unidirectional current flow and consequentlylaser emission in one of multiple linear polarization states. Althoughfour pairs of contacts are illustrated, it will be apparent to thoseskilled in the art that any number of contact pairs may be fabricated,and such modifications are intended to be within the scope of thepresent invention.

[0043]FIG. 5 illustrates an embodiment according to the presentinvention that permits simultaneous emission in multiple polarizationsusing multiple VCSEL cavities that are closely spaced with linearelectrodes corresponding to the separate polarization states. As shownin FIG. 5, multiple pairs of p-n contacts 502A-505A & 502B-505B areplaced around separate, closely spaced current apertures, enablingsimultaneous unidirectional current-flow in each cavity betweencorresponding p-n contacts.

[0044] In another embodiment of the present invention, an exemplarystructure maintains near-constant current in the device with minimalresistive differences between the current paths, which mitigates the RCeffects and achieves polarization switching at high speeds. This designdecouples the operating bias points (and hence optical output power)from the extinction ratio, thereby allowing independent optimization ofeach. Further, to mitigate thermal-heating effects, exemplaryembodiments of the VCSEL structure according to the present inventionmay be operated at a lower than typical threshold, and well belowrollover current. The rollover current depends on the particular VCSELsize; a typical commercial VCSEL may have a threshold of 2 milliamp (mA)and a rollover current of 15 mA. The important point for comparison toaspects of the present invention is that for typical VCSEL currentmodulation (i.e. modulating the intensity of the output light), thespeed of the VCSEL is essentially proportional to the square root of theoperating (i.e., bias) current density minus the threshold currentdensity.

[0045] As an example of the previous discussion, suppose the typicalVCSEL can be operated at a certain speed, X, by using a 3 mA bias, whereI_(bias)−I_(threshold)=1 mA & (1)^(1/2)=1. Then to double the speed ofthe VCSEL to 2X, the bias current would have to be increased to 6 mA,where I_(bias)−I_(threshold)=4 mA & (4)^(1/2)=2. Going one step further,to double the speed again to 4X, would require a bias current to 18 mA,where I_(bias)−I_(threshold)=16 mA & (16)^(1/2)=4 (i.e., but 18 mA isbeyond the 15 mA rollover current). It can be seen then, that as thespeed of the typical VCSEL is increased, the required current quicklygrows, heating up the VCSEL. Thus, the output power and speed does notnecessarily increase by merely increasing the bias current because ofthe accompanying detrimental thermal effects. When doing polarizationmodulation, the speed of the VCSEL is not related to its currentdensity, but rather to how quickly the direction of the polarization ofthe light can be changed, which is related to the direction of currentflow. Therefore, in the present invention, the speed of the VCSEL isdecoupled from the bias current or output power of the VCSEL, and thedetrimental heating effects are be a barrier to increasing speed. In anexemplary arrangement, one embodiment of a VCSEL device according to thepresent invention is operated at a threshold of below 1 mA using anoxide-confined aperture.

[0046] To mitigate mode-competition effects, one arrangement of theinvention provides a single-mode VCSEL with a small aperture (i.e., 5microns or less). When creating a VCSEL with an aperture having alateral extent of between less than about 5 microns, laser emissions ina single transverse mode can be achieved. This single-mode VCSEL hasadvantages in high-speed fiber communication and optical storageapplications. However, single-mode lasers typically have lower poweremissions, and heat up at lower bias currents than do multimode lasers.Further, the single-mode lasers have unproven reliability. By combiningthe concepts of the present invention with a typical small-apertureVCSEL, the advantages of single-emissions are achieved without thenegative heating effects.

[0047] In yet another embodiment of the present invention, the VCSEL isgrown on a suitable substrate determined by the desired emissionwavelength, for instance: at 850 nm, it is typical that the substratewill be GaAs; at 1.55 microns, materials such as InP could be used; at1.3 microns, there are a various choices of materials. In thisembodiment, the substrate also includes a monolithic tuning network toprecisely tune the resistance of the device to approximately 50 Ohms(i.e., 100 Ohms differential). This tuning matches the output impedancesof the VCSEL circuits, which has the beneficial effect of minimizing theelectrical reflections within the device. This monolithic tuning networkcan be fabricated adjacent to, or close to, the VCSEL that it will tune.Other fabrication configurations are possible and are well known in theart. The idea is that the tuning network can be part of the same waferrather than separately fabricated or discretely designed (i.e., designedon board using discrete elements).

[0048] From the foregoing, the present invention can be seen to providea single-mode, current-injected semiconductor laser with high-modulationspeeds (i.e., modulation speeds in excess of 40 Gbit/s), which canapproach fmax of the VCSEL. The present invention allows link operationin single-ended mode by converting polarization modulation to intensitymodulation; alternatively, it allows link operation naturally indifferential mode (i.e., if both polarizations are carried to separatereceivers). Further, the VCSEL of the present invention can be usefulin, for example, optical communication, optical imaging and opticalsensing applications.

[0049] As discussed, an embodiment of the VCSEL according to the presentinvention entails the use of two pairs of orthogonal lateral-injectioncontact electrodes with a single VCSEL cavity as a means forpolarization switching via current steering. This design decouples theoperating bias points (and hence optical output power) from theextinction ratio, thereby allowing independent optimization of each. Inan exemplary arrangement (as previously described in relation to FIGS.3A-B), the contact pairs are arranged on opposite sides of the VCSELcavity so as to allow uniaxial flow of current across the cavity.Polarization switching is achieved by changing the direction of thisuniaxial current flow. To mitigate RC effects and to achievepolarization switching at high speeds, an exemplary driver circuit forthe VCSEL of the present invention should maintain near-constant currentin the device with minimal resistive differences between the currentpaths.

[0050]FIG. 6 shows a generalized driver circuit arrangement according toan embodiment of the present invention. As shown in FIG. 6, a controlcircuit is used to supply current under the control of a tuning voltage(V_(control)) into the polarization-switching VCSEL. The current isswitched between two independent branches (p1-n1 and p2-n2) of the VCSELusing a switching circuit under the control of the input switchingvoltage (V_(switch)). In order to achieve correct operation of thedisclosed VCSEL, current should flow between each designated pair ofelectrodes on opposite sides of the cavity and current should not flowbetween neighboring electrodes. To facilitate this, the driver circuitshould break the degeneracy between the two pairs of contact electrodesto prevent forbidden (i.e., stray) currents from flowing. Severalspecific embodiments of polarization-switching VCSEL driver circuits arenow presented to exemplify the driver circuit concepts described.

[0051]FIG. 7 shows a first VCSEL driver circuit embodiment 500 of theinvention. For the purposes of FIG. 7, the horizontal polarizationemission 100 and the vertical polarization emission 102 are shownseparately. The p-contact of the respective orthogonal contactelectrodes are attached to separate voltage supplies V_(dd1) 101 andV_(dd2) 103, respectively. Transistors 110 and 112 provide a mirroredadjustable bias current I_(bias1) 111 to p-polarized laser lightemission. Transistors 114 and 116 correspondingly provide a mirroredadjustable bias current I_(bias2) 115 to h-polarized emissions. Thetransistors 104 and 106 represent a differential pair driven by digitalinputs IN1 105 and IN2 107 that steer the adjustable modulation currentI_(mod) 109 generated by transistors 108 and 110 between the twobranches of the circuit. The circuit uses a common ground line 117.During operation of the driver circuit 500, current is steered betweenthe two independent branches of the VCSEL under the control ofdifferential inputs. The separate supply voltages are used to isolatethe p-contacts of the VCSEL. The circuit 500 provides independent biascurrents for each polarization component of the VCSEL.

[0052]FIG. 8 shows a second VCSEL driver circuit embodiment 600 of theinvention. As shown in FIG. 8, a variation to the circuit described inFIG. 8 would be to use a common positive supply line V_(dd) 125 withseparate negative supply lines V_(ss1) 133 and V_(ss2) 135 connected,respectively, to the n-contacts of the horizontal polarized emission 120and vertical polarized emission 122. The respective bias currents foreach branch of the circuit, I_(bias1) and I_(bias2), can be generated bya simple voltage controlled current sources 124 and 126, respectively.The transistors 128 and 130 represent a pair driven by digital inputsIN1 and IN2 that steer the adjustable modulation current I_(mod)generated by transistor 132 between the two branches of the circuit.Alternately, I_(bias1) and I_(bias2) could be generated by mirrorcurrent sources as in FIG. 7.

[0053] Other variations of these two driver circuits can be easilyimagined by those skilled in the art; such variations are intended to bewithin the scope of the present invention. For example, in FIG. 7,adjustable bias currents I_(bias1) 111 and I_(bias2) 115 can be replacedwith a digital-to-analog converter for precise external digital control.Although FIG. 7 is illustrated using CMOS transistors, the transistorscould easily be replaced with bipolar junction transistors for certainhigh-speed applications. Additionally, discrete components could be usedwhere integrated circuits have been shown. Further, fixed differentialinput transistors 104 and 106 could be replaced with a bank of differentsized transistors chosen to optimize the current steering speed based onthe value of I_(mod). Corresponding variations for the circuit in FIG. 8could be used.

[0054]FIG. 9 shows a third VCSEL driver circuit embodiment 700 of theinvention. FIG. 9 illustrates a driver circuit technique to break thedegeneracy of the contacts and achieve proper circuit operation byproviding current switching transistors for both the positive andnegative supply lines. In this variation of the original circuit shownin FIG. 7, pass transistors 146 and 146 are inserted between a commonpositive supply line V_(dd) and the p-polarized and h-polarizedemissions, respectively. Pass transistor 144 serves to enable current toflow in the left branch of the circuit, as shown, when input IN1 is highand IN2 is low, while pass transistor 146 simultaneously stops currentflow in the right branch of the circuit. Vice-versa, when IN1 is low andIN2 is high, pass transistor 144 prevents the left-hand side of thecircuit from drawing current, whereas pass transistor 146 allows currentto flow into the right-hand side. This removes VCSEL contact degeneracywhile using a common supply voltage line V_(dd).

[0055]FIG. 10 shows a fourth VCSEL driver circuit embodiment 800 of theinvention. FIG. 10 shows a driver circuit derived from FIG. 8, using thetechnique of FIG. 9, above. As shown in FIG. 10, pass transistors 180and 182 are inserted between a common ground line V_(ss) and thep-polarized 164 and h-polarized 166 emissions, respectively. When IN1 ishigh and IN2 is low, pass transistor 180 shutters on and pass transistor182 respectively shutters off, enabling current to flow into thep-polarized VCSEL contacts 164 and simultaneously preventing currentfrom flowing into the h-polarized VCSEL contacts 166. Vice-versa, whenIN2 is high and IN1 is low, the pass transistors 180 and 182respectively allow current to flow into the h-polarized VCSEL contacts166 and prevent current flow into the p-polarized contacts 164.Corresponding circuit variations for generating the I_(bias) currentsare possible for the circuits of FIGS. 9 and 10 as discussed previouslyfor the circuits of FIGS. 7 and 8.

[0056] Although the present invention has been particularly describedwith reference to the preferred embodiments thereof, it should bereadily apparent to those of ordinary skill in the art that changes andmodifications in the form and details thereof may be made withoutdeparting from the spirit and scope of the invention. For example, thoseskilled in the art will understand that variations can be made in thenumber and arrangement of components and contacts illustrated in thefigures. Further, while the VCSEL structure of the exemplary figures isshown to be circular, it should be understood that any geometric shapecan be fabricated as part of the present invention. It is intended thatthe appended claims include such changes and modifications.

What is claimed is:
 1. A vertical cavity surface emitting laser (VCSEL)capable of polarization switching, comprising: a substrate; a bottommirror disposed on the substrate; an active region disposed on thebottom mirror, the active region including a current aperture and aVCSEL cavity, wherein an active region size is smaller than a bottommirror size; a first pair of bottom electrodes disposed on the bottommirror near a bottom periphery of the active region; and a first pair oftop electrodes disposed on the active region near a top periphery of theactive region.
 2. The VCSEL of claim 1, wherein the bottom mirror is asemiconductor distributed Bragg reflector mirror.
 3. The VCSEL of claim1, wherein the active region size is between approximately 3 and 20microns in diameter.
 4. The VCSEL of claim 1, wherein: the first pair ofbottom electrodes are disposed approximately 90 Degrees from one anotheraround the bottom periphery; the first pair of top electrodes aredisposed approximately 90 Degrees from one another around the topperiphery; one of the first pair of bottom electrodes is disposedapproximately 180 Degrees from one of the first pair of top electrodes;and another of the first pair of bottom electrodes is disposedapproximately 180 Degrees from another of the first pair of topelectrodes.
 5. The VCSEL of claim 4, further comprising a drivercircuit, the driver circuit including: means for supplying one or morefirst supply energies to the one of the first pair of bottom electrodesthat is disposed approximately 180 Degrees from the one of the firstpair of top electrodes; means for supplying one or more second supplyenergies to the another of the first pair of bottom electrodes that isdisposed approximately 180 Degrees from the another of the first pair oftop electrodes; means for controlling the one or more first and secondsupply energies; and means for switching the between the one or morefirst and second supply energies.
 6. The VCSEL of claim 1, furthercomprising a top mirror disposed on the active region.
 7. The VCSEL ofclaim 6, wherein the top mirror is a dielectric distributed Braggreflector having a top mirror thickness of less than 3 micron.
 8. TheVCSEL of claim 7, wherein the top mirror thickness is 1 micron or less.9. The VCSEL of claim 1, further comprising: a second pair of bottomelectrodes disposed on the bottom mirror near the bottom periphery ofthe active region; and a second pair of top electrodes disposed on theactive region near the top periphery of the active region.
 10. The VCSELof claim 9, wherein: the first and second pairs of bottom electrodes areall disposed approximately 90 Degrees from one another around the bottomperiphery; the first and second pairs of top electrodes are all disposedapproximately 90 Degrees from one another around the top periphery; oneof the first and second pairs of bottom electrodes is disposedapproximately 180 Degrees from one of the first and second pairs of topelectrodes, respectively; and another of the first and second pairs ofbottom electrodes is disposed approximately 180 Degrees from another ofthe first and second pairs of top electrodes, respectively.
 11. TheVCSEL of claim 10, further comprising a driver circuit, the drivercircuit including: means for supplying one or more first supply energiesto the one of the first and second pairs of bottom electrodes that isdisposed approximately 180 Degrees from the one of the first and secondpairs of top electrodes; means for supplying one or more second supplyenergies to the another of the first and second pairs of bottomelectrodes that is disposed approximately 180 Degrees from the anotherof the first and second pairs of top electrodes; means for controllingthe one or more first and second supply energies; and means forswitching between the one or more first and second supply energies. 12.A multi-cavity vertical cavity surface emitting laser (VCSEL) capable ofsimultaneous multiple polarization emissions, comprising: a substrate; abottom mirrors disposed on the substrate; a plurality of active regionsdisposed on the bottom mirror, each active region including a currentaperture and a VCSEL cavity, wherein a combined active region size ofthe plurality of active regions is smaller than a bottom mirror size; aplurality of bottom electrodes disposed on the bottom mirror, eachbottom electrode being positioned near a bottom periphery of a differentone of the plurality of active regions; and a plurality of topelectrodes disposed on the active region, each top electrode beingpositioned near a top periphery of the different one of the plurality ofactive region, wherein: each bottom electrode and each top electrode arepositioned approximately 180 Degrees from one another around thedifferent one of the plurality of active regions.
 13. The multi-cavityVCSEL of claim 12, further comprising a driver circuit, the drivercircuit including: means for supplying a plurality of supply energies toeach bottom electrode and each top electrode that are positionedapproximately 180 Degrees from one another around the different one ofthe plurality of active regions; means for controlling the plurality ofsupply energies; and means for switching the between the plurality ofsupply energies.
 14. A method of operating a vertical cavity surfaceemitting laser (VCSEL) for polarization switching comprising the stepsof: providing a VCSEL, the VCSEL including: a substrate; a bottom mirrordisposed on the substrate; an active region disposed on the bottommirror, the active region including a current aperture and a VCSELcavity, wherein an active region size is smaller than a bottom mirrorsize; a first pair of bottom electrodes disposed on the bottom mirrornear a bottom periphery of the active region; and a first pair of topelectrodes disposed on the active region near a top periphery of theactive region; switching one or more supply energies between a firstelectrically opposed pair of the top and bottom electrodes and a secondelectrically opposed pair of the top and bottom electrodes.
 15. Themethod of claim 14, wherein the bottom mirror is a semiconductordistributed Bragg reflector mirror.
 16. The method of claim 14, whereinthe active region size is between approximately 3 and 20 microns indiameter.
 17. The method of claim 14, wherein: the first pair of bottomelectrodes are disposed approximately 90 Degrees from one another aroundthe bottom periphery; the first pair of top electrodes are disposedapproximately 90 Degrees from one another around the top periphery; oneof the first pair of bottom electrodes is disposed approximately 180Degrees from one of the first pair of top electrodes; and another of thefirst pair of bottom electrodes is disposed approximately 180 Degreesfrom another of the first pair of top electrodes.
 18. The method ofclaim 17, wherein the step of switching includes providing a drivercircuit, the driver circuit including: means for supplying one or morefirst supply energies to the one of the first pair of bottom electrodesthat is disposed approximately 180 Degrees from the one of the firstpair of top electrodes; means for supplying one or more second supplyenergies to the another of the first pair of bottom electrodes that isdisposed approximately 180 Degrees from the another of the first pair oftop electrodes; means for controlling the one or more first and secondsupply energies; and means for switching the between the one or morefirst and second supply energies.
 19. The method of claim 14, whereinthe VCSEL further includes a top mirror disposed on the active region.20. The method of claim 19, wherein the top mirror is a dielectricdistributed Bragg reflector having a top mirror thickness of less than 3micron.
 21. The method of claim 20, wherein the top mirror thickness is1 micron or less.
 22. The method of claim 14, wherein, the VCSEL furtherincludes: a second pair of bottom electrodes disposed on the bottommirror near the bottom periphery of the active region; and a second pairof top electrodes disposed on the active region near the top peripheryof the active region; and the step of switching further includes:switching another one or more supply energies between a firstelectrically opposed pair of the second pair of top and bottomelectrodes and second electrically opposed pair of the second pair oftop and bottom electrodes.
 23. The method of claim 22, wherein: thefirst and second pairs of bottom electrodes are disposed approximately90 Degrees from one another around the bottom periphery; the first andsecond pairs of top electrodes are disposed approximately 90 Degreesfrom one another around the top periphery; one of the first and secondpairs of bottom electrodes is disposed approximately 180 Degrees fromone of the first and second pairs of top electrodes, respectively; andanother of the first and second pairs of bottom electrodes is disposedapproximately 180 Degrees from another of the first and second pairs oftop electrodes, respectively.
 24. The method of claim 23, wherein thestep of switching includes providing a driver circuit, the drivercircuit including: means for supplying one or more first supply energiesto the one of the first and second pairs of bottom electrodes that isdisposed approximately 180 Degrees from the one of the first and secondpairs of top electrodes; means for supplying one or more second supplyenergies to the another of the first and second pairs of bottomelectrodes that is disposed approximately 180 Degrees from the anotherof the first and second pairs of top electrodes; means for controllingthe one or more first and second supply energies; and means forswitching the between the one or more first and second supply energies.25. A method of operating a vertical cavity surface emitting laser(VCSEL) for simultaneous multiple polarization switching comprising thesteps of: providing a VCSEL, the VCSEL including: a substrate; a bottommirrors disposed on the substrate; a plurality of active regionsdisposed on the bottom mirror, each active region including a currentaperture and a VCSEL cavity, wherein a combined active region size ofthe plurality of active regions is smaller than a bottom mirror size; aplurality of bottom electrodes disposed on the bottom mirror, eachbottom electrode being positioned near a bottom periphery of a differentone of the plurality of active regions; and a plurality of topelectrodes disposed on the active region, each top electrode beingpositioned near a top periphery of the different one of the plurality ofactive region, wherein each bottom electrode and each top electrode arepositioned approximately 180 Degrees from one another around thedifferent one of the plurality of active regions; and switching one ormore supply energies between a first electrically opposed pair of theplurality of top and bottom electrodes and all other electricallyopposed pairs of the plurality of top and bottom electrodes.
 26. Themulti-cavity VCSEL of claim 25, further comprising a driver circuit, thedriver circuit including: means for supplying a plurality of supplyenergies to each bottom electrode and each top electrode that arepositioned approximately 180 Degrees from one another around thedifferent one of the plurality of active regions; means for controllingthe plurality of supply energies; and means for switching the betweenthe plurality of supply energies.
 27. A vertical cavity surface emittinglaser (VCSEL) driver circuit capable of polarization switching a VCSEL,the VCSEL including a plurality of pairs of polarizing electricalcontacts, comprising: means for supplying one or more supply energies tothe plurality of pairs of polarizing electrical contacts of the VCSEL;means for controlling the one or more supply energies; and means forswitching the one or more supply energies between the plurality of pairsof polarizing electrical contacts of the VCSEL.